Horse Plow, Horse Collar, Horseshoes
The Beginning of the Knights 

Around 950 CE, three new inventions changed the way people worked the fields. 

The Plow: A new way to farm was invented. The new plow dug deep into the soil. A plowed field was a line of deep furrows with soil piled up on one side. Seed was planted and then covered with the soil. Water ran along the furrows, which irrigated the crops. Crops grew. 

The Horse Collar: A horse can pull weight than an ox, but to do so, horses need a special collar to protect their neck. The horse collar was invented around the same time as the plow. That did the trick.

Horseshoes: The horseshoe was invented to help valuable horse work more effectively.

The horse-drawn plow changed everything. It took less workers to grow more crops. Even after storing ample amounts of food to feed everyone on the fief all year long, there was enough food left over to support small armies. These new well-equipped, well-trained professional warriors were called knights. 

You might hear people refer to any fighting men during the middle ages as knights. But the knights of legend did not appear until the invention of the horse-drawn plow.